1. Field of the Invention
The present invention relates to a communication system. More particularly, it relates to an improved channel switching system without instantaneous signal loss during switching between an active transmission channel and a standby transmission channel in a communication system.
2. Description of the Related Art
In a communication system such as a digital microwave multiple radio communication system, a standby transmission channel is provided in addition to active transmission channels to switch thereto from the active transmission channel in which deterioration of a transmission performance (quality) due to fading or a fault due to, for example, a fault in a radio communication apparatus, i.e. a transmitter and/or a receiver. Accordingly, a channel switching system is provided in each active transmission channel, to enable a switch-over between the active transmission channel and the standby transmission channel.
In a prior art, a mercury coaxial relay is used for the switching means. The mercury coaxial relay has an advantage of providing excellent isolation between the active transmission channel and the standby transmission channel, while maintaining a low signal leakage therebetween. The mercury coaxial relay also prevents the generation of a mismatch of impedances on the transmission channels and has the high reliability. However, the mercury coaxial relay has a disadvantage of a low speed operation time, for example, several milliseconds, and the contacts thereof are fully open for a short period during switching. As a result, an instantaneous signal loss cannot be avoided. In addition, the switching is carried out unconditionally. In other words, a phase synchronization between the active transmission channel and the standby transmission channel cannot be made, and accordingly, a burden of frame synchronization at a carrier terminal connected to the channel switching system is increased. Therefore, until the phase is adjusted, an actual signal loss of approximately several hundreds milliseconds may occur.
Japanese Patent Publication No. 57-58419, entitled "A Transmission Line Switching System Without Instantaneous Loss", and published on Apr. 4, 1982, solves the above defects by providing a high speed switching means, such as a semiconductor switch, in addition to the mercury relay, and employs a phase pattern matching and adjustment procedure prior to the channel switching. The operation of the switching system will be described later in detail with reference to the drawings.
The switching system achieves a good switching operation with regard to combatting fading, but suffers from the following defects. First, when a device in the active transmission channel malfunctions, signals on the active transmission channel are eliminated, and accordingly, the pattern matching and phase adjustment of signals between the active transmission channel and the standby transmission channel cannot be performed satisfactorily, thus disabling the channel switching by the high speed switching means. At this time, channel switching by the mercury relay can be attempted, and when the active transmission channel is restored, a channel switch-back from the standby transmission channel to the active transmission channel can be achieved by the mercury relay. However, at this time, an instantaneous signal loss occurs to cause an out of phase signal at the carrier terminal, as in the above prior art. Second, when the standby transmission channel malfunctions after the channel is switched from the active transmission channel to the standby transmission channel by the high speed switching means, signals on the standby transmission channel are eliminated. If the active transmission channel is restored in this condition, signal patterns therebetween do not coincide. As a result, a channel switching-back from the standby transmission channel to the active transmission channel cannot be carried out, and thus the channel malfunction state continues.